For the first time ever the H5N9 avian influenza virus has caused an outbreak in the U.S. This ... [+] occurred on a duck farm in Merced, California. Pictured here is a random duck, not from that farm. (Photo: Getty)gettyA bird in hand may be worth two in the bush. But two bird flu strains out of hand would certainly be worse than one. The U.S. hasnt been able to control the spread of the H5N1 bird flu. And now another strain of avian influenza, H5N9, has caused for the first time ever an outbreak in the U.S.This H5N9 outbreak occurred on a duck farm in Merced, California, and has been reported to the World Organisation for Animal Health, known as WOAH. And whoa, the outbreak resulted in nearly 119,00 birds perishing as of December 2, 2024, although its not clear how many of these birds died from infections versus were culled to prevent further spread of H5N9.What Makes H5N9 Different From H5N1When you are scrolling through the news or social media and only half paying attention, it may be easy to mistaken H5N9 for H5N1 since the only difference in their names is the number following the N. But these are two different lineages that in turn have their own sets of different clades and subclades under them. In other words, there are lots of different H5N1 viruses and lots of different H5N9 viruses. In this case, H stands for hemagglutinin, a protein that is found on the surface of influenza type A viruses that helps get the virus into cells that it infects and can come in 18 different subtypes: H1 through H18. The N stands for neuraminidase, a different protein that helps the virus out of cells and can come in 11 different subtypes: N1 through N11.This H5N1 Strain Has Been Spreading Further For A Longer Time Than The H5N9 StrainHaving to worry now about two different strains of highly pathogenic avian influenza isnt exactly a two tickets to paradise situation. But could it be two possible tickets to a pandemic? Well, the H5N1 strain has been spreading among birds for the past several years and recently appeared in other animals like cattle, cats, pigs and, yes, humans, as Ive described for Forbes. At least 67 humans have tested positive for the H5N1 strain to date with one person in Louisiana dying, as reported by the Louisiana Department of Public Health.The positive news is that there hasnt yet been any clear evidence of any of these 67 people catching the virus from another human. But the more this strain continues to spread the greater the concern that H5N1 may eventually acquire the ability to spread among humans, which in turn would raise the possibility of the p word.MORE FOR YOUA pandemic is when a novel pathogenmeaning one that hasnt really infected humans previouslyspreads among humans in different countries simultaneously, as I have described for Forbes. Novel pathogens can wreak havoc because your immune system can behave like a virgin on a first date when encountering them. It can fire off in random directions, trying all sorts of things that wont work and in fact cause more self-harm. Thats what happened during the 1918 H1N1 influenza pandemic and the COVID-19 pandemic.Mutations And Reassortments Could Change The Pandemic Potential Of H5N9So far, the H5N9 strain hasnt seemed to spread nearly as far as the H5N1 strain. Therefore, at this moment, the H5N9 doesnt seem to be as big a p threat as the H5N1. Keep in mind, though, that bird flu strains are sort of like celebrity marriages. You never know what may happen and what is currently happening behind the scenes. You never know when a series of mutations or genetic reassortments in the virus could significantly change what it can do. Reassortment is when two or more influenza viruses infect a single host and then swap segments of their genetic material with each other.The more birds get infected with the H5N1 or N5N9 viruses, the more likely it is for different mutations and reassortments to occur. Chances of such chance events increase even more when other animals get infected like pigs where reassortments are even more likely. Thats why its important to control the spread of such viruses among birds even if you are not a bird. And why the failure of the U.S. to do so could end up endangering all of us. After all, you never know when mutations and reassortments could end giving humans the bird, the bird flu that is.